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Also the hole you can see in the lower part of the saucer suggests that this is a piston design while the USD JMC RRBut it still doesn't match the photo the OP posted (finish, port plugs, environmental seal/cap, yoke, etc.):
Also the hole you can see in the lower part of the saucer suggests that this is a piston design while the USD JMC RRis a diaphragm. Although I guess it could be just a spot to insert a hook spanner for disassembly since it doesn't appear to have wrench flats like the USD reg.
Liklely mid 80's Spiro, before Aqualung consolidated brand names.Any ideas what reg this is? I was thinking AL but cant find a schematic that fits that shape.